Chromogenic substrate

ABSTRACT

The invention relates to, novel peptide derivatives with the formula: R 1  -A 1  -A 2  -A 3  -A 4  -R 2  or its salt, wherein R 1  =H or a protective group, A 1  =H, Ile, Leu or Val, A 2  =Glu, Asp, Ser, Thr; A 3  =Gly or Glyc; A 4  =Arg or Lys; R 2  =4-nitroaniline with the proviso that A 3  is Gly when A 4  is Lys and that A 3  is Glyc when A 4  is Arg. The invention also discloses the process for the preparation of the peptide derivatives, the method for the determination of the bacterial endotoxins by the use of the invented peptide derivatives and the use of these derivatives for determination of bacterial endotoxins.

FIELD OF THE INVENTION

The present invention relates to a new chromogenic synthetic substrate used for quantitative determination of bacterial endotoxins in physiological fluids, food, pharmaceuticals etc.

THE BACKGROUND OF THE INVENTION

Bacterial endotoxins are produced by Gram negative bacteria and are considered by most investigators to be a very important factor in the development of septecemia. Several methods for determination of endotoxin have been described based on the observation by Levin and Bang (1956)that endotoxins specifically activate the clotting system of Limulus Potyphemus. In the beginning a test, in which Limulus Amebocyte Lysate (LAL)in contact with endotoxins containing sources, produce a specific gelation, had been developed. More recently chromogenic and fluorogenic methods based on the above observation have been developed and can rapidly detect small amounts of endotoxins (H. C. Hemker: Handbook of Synthetic Substrates, 1983, Martinus Nijhoff Publisher, Boston).

PRIOR ART

A. E. Torano et al have disclosed that an enzyme from Limulus amebocyte lysate shows similar specificity to mammalian blood coagulation factor X_(a) (Thrombosis Research 34, 407-417, 1984), which recognize the sequence -Ile-Glu-Gly-Arg- in its natural substrate prothrombin.

Other investigations have also Shown the important role the COOH-terminal Gly-Arg sequence plays when analysing endotoxin. T. Harada et al, Biomedical Applications of the Horseshoe Crab (Limulidal), E. Cohen (ed), Alan R. Liss Inc. New York, 1979, pages 209-220, disclose in a table on page 213 different substrates which are used, but only those having the carboxy termial sequence Gly-Arg give interesting results. On page 212, lines 7-10 from the bottom, the authors point out: "These results clearly indicate that Limulus clotting enzyme displays a high specificity towards the peptide pNA having COOH-terminal Gly-Arg sequence".

U.S. Pat. Nos. 4,188,264 and 4,576,745 also give Gly-Arg as carboxy-terminal sequence in the substrate used for determination of endotoxin. Other substrates have been investigated, all of them showing Arg as carboxy terminal.

According to U.S. Pat. No. 4,406,832 the carboxy terminal should be -Ala-Arg- or -Cys-Arg-and this substrate has given relative activity which is as good as or a little better than standard -Gly-Arg-. Due to the complicated mechanism in these reactions it is not possible to know which peptide sequence could give an acceptable result.

Surprisingly and against the prior art within the field, we have now found that a substrate which has the carboxy terminal sequence -Gly-Lys- gives a relative activity which is at least 20% better compared with known substrates.

The use of Glycolic acid (Glyc) in a substrate for determination of endotoxin has never been disclosed before, and it is very surprising that the effect is as good when using -Glyc-Arg- as when using Gly-Lys, both of them giving a better effect than the normally used Gly-Arg.

DESCRIPTION OF THE INVENTION

The chromogenic synthetic peptide or peptide isostere derivative in the present invention shows high sensitivity in the method used for the determination of endotoxins.

The new substrates are characterized by the following formula:

    R.sub.1 -A.sub.1 -A.sub.2 -A.sub.3 -A.sub.4 -R.sub.2 or its salt

where

R₁ =H or a protective group

A₁ =H, Ile, Leu or Val

A₂ =Glu, Asp, Ser, Thr

A₃ =Gly or Glyc

A₄ =Arg or Lys

R₂ =4-nitro aniline

with the proviso that A₃ is Gly when A₄ is Lys and that A₃ is Glyc when A₄ is Arg.

The present invention also discloses the process for the preparation of the peptid derivatives.

It also discloses the method for determination of bacterial endotoxins by the use of the derivative with the following formula:

    R.sub.1 -A.sub.1 -A.sub.2 -A.sub.3 -A.sub.4 -R.sub.2 or its salt

where

R₁ =H or a protective group

A₁ =H, Ile, Leu or Val

A₂ =Glu, Asp, Ser, Thr

A₃ =Gly or Glyc

A₄ =Arg or Lys

R₂ =an aromatic or heterocyclic group which gives the compound

R₂ -NH₂ by enzymatic hydrolysis

R₂ -NH₂ are prior known compounds with chromogenic properties which permit quantifying of endotoxins by determination of splitted marker directly or after derivatization (H. C. Hemker: Loc. cit.) with the proviso that A₃ is Gly when A₄ is Lys and that A₃ is Glyc when A₄ is Arg.

The use of these derivatives for determination of bacterial endotoxins is also disclosed.

Example of compounds which could be R₂ -NH-₂ are: p-nitroaniline, 3-carboxy-4-hydroxyaniline, 3-sulfo-4-nitroaniline, 3-alkoxy-4-nitroaniline, 3-carboxy-4-nitroaniline, 4-methyloxy-naphtylamine, 4-(N-ethyl-N-hydroxyethyl) aminoaniline, 5-amino-isophtalic acid-dimethyl ester, 5-amino-8-nitroquinoline, 7-amino-4-trifluormethyl coumarine, 7-amino-4-methyl coumarine, 4-aminodiphenylamine. The invention also discloses the use of these derivatives for determination of bacterial endotoxins.

The new peptide or peptide isostere derivatives contain as carboxyterminal Lys when combined with Gly and Arg when combined with Glyc. The combination Gly-Lys was considered unfavourable until present in substrates used for Limulus amebocyte lysate and the combination Glyc-Arg has never been used in this type of substrates before in methods for determination of bacterial endotoxins.

Description of Synthesis

Conventional techniques for coupling and conventional protecting groups (Z, Boc etc) used within the peptide chemistry (M. Bodanzsky: Principles of Peptide Synthesis, Springer Verlag 1984) e.g. addition step-by-step of the amino acids at the C-terminal amino acid provided with a marker or synthesis of the N-terminal peptide fragment per se, which then is coupled to the C-terminal amino acid provided with a marker, have been used.

The synthesis of different substrates according to the invention will be described more in detail in the following not limited working examples.

Purification of the intermediates and end products were performed by precipitation, crystallization or gel filtration chromatography. The purified end products were lyophilized. Prefabricated glass plates of silicagel F₂₅₄ were used for TLC analyses. After terminated chromatography the plates were inspected in U.V. light (254 nm) and were developed thereafter with ninhydrine and chlorine/dicarboxidine reagent. The Rf value given are results from single chromatographies.

Used solvent system for TLC have been indicated according to the following table:

    ______________________________________                                         Indication                                                                              Solvent system       Volume ratio                                     ______________________________________                                         A        n-butanol:AcOH:water 3:2:1                                            Pa.sub.6 chloroform:MeOH:AcOH:water                                                                          34:4:9:2                                         P1       chloroform:MeOH      9:1                                              Pa       chloroform:MeOH:AcOH 17:2:2                                           ______________________________________                                    

HPLC analysis were performed on Merck R. P. column (Hibar Lichracart) with 40% MeOH in 5% triethylaminophosphate pH 2.35 as eluent (1 ml/min). The optical activity of the end products were determined at 589 nm in 50% AcOH at a concentration of 0.4-1.0 g/100 ml at 25° C. The below mentioned abbreviations have the following meaning: (I.U.P.A.C. indication has been used were such exists).

    ______________________________________                                         Amino acids:                                                                   ______________________________________                                         Arg = arginine    Val = valine                                                 Gly = glycine     Glu = glutamic acid                                          Ile = isoleucine  Asp = aspartic acid                                          Leu = leucine     Ser = serine                                                                   Thr = threonine                                              ______________________________________                                    

All amine acids in the substrates have L-configurattion if not else is indicated.

The free amine acid or peptide is indicated by H- at the N-terminal amine group and --OH at the carboxy terminal group. The amine group is always given to the left and the carboxy to the right.

    __________________________________________________________________________     Abbreviations:                                                                 __________________________________________________________________________     Ac  = acetyl         Et.sub.3 N                                                                         = triethylamine                                       AcOH                                                                               = acetic acid    Glyc                                                                               = glycolic acid                                       AMC = 7-amino-4-methylcoumarine                                                                     HOBT                                                                               = 1-hydroxybenzotriazol                               Boc = t-butyloxycarbonyl                                                                            HPLC                                                                               = high performance liquid                             Bz  = benzoyl            .sup.   chromatography                                Bzl = benzyl         MeOH                                                                               = methanol                                            DCCI                                                                               = dicyclohexylcarbodiimide                                                                      ONp = nitrophenyl ester                                   DCU = dicyclohexylurea                                                                              OSu = hydroxysuccinimide                                  DMAP                                                                               = dimethylaminopyridine                                                                             .sup.   ester                                         DMF = dimethylformamide                                                                             pNA = p-nitroaniline                                      EtOAc                                                                              = ethylacetate   TFA = trifluoroacetic acid                                EtOH                                                                               = ethanol        Z   = benzyloxycarbonyl                                   __________________________________________________________________________

EXAMPLE 1

    ______________________________________                                         α-Ac--Ile--Glu--Gly--Lys--pNA.HCl                                                             molecular weight =                                                             644.15                                                    Z--Lys--pNA.TFA      Molecular weight =                                                             514.5                                                     ______________________________________                                    

9 ml TFA is added to 10 mmol α-Boc-ε-Z-Lys-pNA dissolved in 25 ml methylenchloride. The solution is stirred for 30 minutes at room temperature and is precipitated with a mixture 2:1 of t-butylmethylether-petroleumether.

Yield 95%.

TLC: Rf=0.60 (A).

1b) α-Boc-γ-O-Bzl-Glu-Gly-OH Molecular weight=394.4

To 0.1 mol H-Gly-OH dissolved in 200 ml 1 molar NaHCO₃ a solution of 0.1 mol of α-Boc-γ-O-Bzl-Glu-O-Su in 200 ml dioxan is added. The mixture is stirred overnight at room temperature. Next day the solution is evaporated in vacuo to an oily residue which is dissolved in 100 ml water and washed with diethylether. The water phase is brought to pH 2 with KHSO₄ solution and extracted with EtOAc. After drying with Na₂ SO₄, the EtOAc solution is evaporated and the substance precipitated with a mixture (1:1) of diethylether-petroleumether.

Yield 85%.

TLC: Rf=0.5 (Pa).

1c) H-γ-O-Bzl-Glu-Gly-OH.TFA Molecular weight=408.4

9 ml TFA is added to 10 mmol α-Boc-γ-O-Bzl-Glu-Gly-OH (1b) dissolved in 20 ml methylenchloride. The mixture is stirred for 30 minutes at room temperature, evaporated in vacuo to an oil and precipitated with diethylether.

Yield: 90%

TLC: Rf=0.44 (A).

1d) α-Boc-Ile-γ-O-Bzl-Glu-Gly-OH Molecular weight=507.6

To 0.1 mol H-γ-O-Bzl-Glu-Gly-OH.TFA dissolved in 250 ml 1 molar NaHCO₃ a solution of 0.1 mol α-Boc-Ile-O-Su in 250 ml dioxan is added. The mixture is stirred overnight at room temperature. Next day the solution is evaporated in vacuo, the residue dissolved in 100 ml water and washed with EtOAc. The water phase is brought to pH 3 with KHSO₄ solution and extracted with EtOAc. After drying with Na₂ SO₄ the EtOAc solution is evaporated and the substance precipitated with petroleumether.

Yield: 92%

TLC: Rf=0.6 (Pa)

1e) α-Boc-Ile-γ-O-Bzl-Glu-Gly-ε-Z-Lys-pNA Molecular weight=890.0

3 mmol ε-Z-Lys-pNA.TFA (1a) dissolved in 25 ml DMF is neutralized in cold (-10° C.) with Et₃ N. To the solution 3 mmol α-Boc-Ile-γ-O-Bzl-Glu-Gly-OH (1d), 3 mmol HOBT and 3.2 mmol DCCl are added. The mixture is stirred for 1 hour in cold and overnight at room temperature. The formed DCU is filtered off and the solution is evaporated in vacuo to an oil which is dissolved in EtOAc and is washed with 2% NaHCO₃, 2% KHSO₄ and H₂ O. After drying with with Na₂ SO₄, the EtOAc phase is evaporated and the substance is precipitated with diethylether.

Yield: 61%

TLC: Rf=0.67 (P₁)

1f) α-Ac-Ile-γ-O-Bzl-Glu-Gly-ε-Z-Lys-pNA Molecular weight=831.9

3.5 ml TFA is added to 1.2 mmol α-Boc-Ile-γ-O-Bzl-Glu-Gly-ε-Z-Lys-pNA (1e) in 6 ml methylenchloride. The mixture is stirred for 30 minutes at room temperature and is precipitated with diethyleter. The dry substance is dissolved in 10 ml DMF and neutralized in cold (-10° C.) with 160 μl Et₃ N. 140 μl acetic anhydride is added and the mixture is stirred for 1 hour in cold and 2 hours at room temperature. The solution is evaporated in vacuo to an oil and the substance is precipitated with water.

Yield: 86%

TLC: Rf=0.45 (P1).

1) α-Ac-Ile-Glu-Gly-Lys-pNA.HCl Molecular weight=644.15

10 ml triflic acid is added to a cold (-10° C.) suspension of 1 mmol α-Ac-Ile-γ-O-Bzl-Glu-Gly-Z-Lys-pNA-(1f) in 10 ml methylenchloride. The mixture is stirred for 50 minutes at room temperature and precipitated with diethylether. The dried substance is ion exchanged on a Sephadex® QAE-25 column, in chloride form with 50% ETOH as eluent and is purified on a Merck Lobar® prepacked column (Lichroprep.® RP-8-B) with 50% MeOH as eluent (2 ml/minute). The purified product is lyophilized.

Yield: 42%

TLC: Rf=0.2 (Pa₆)

HPLC: 98% purity

[α]_(D) ²⁵ =-63.1° (C=0.5%)

EXAMPLE 2

α-Ac-Ile-Ser-Gly-Lys-pNA.HCl Molecular weight=602.11

2a) α-Boc-Gly-ε-Z-Lys-pNA Molecular weight=557.6

5 mmol ε-Z-Lys-pNA.TFA (prepared as described in example 1a) dissolved in 25 ml DMF is neutralized in cold (-10° C.) with Et₃ N. To the solution 5 mmol α-Boc-Gly-OH, 5 mmol HOBT and 5 mmol DCCl are added. The mixture is stirred for 1 hour in cold and overnight at room temperature. The formed DCU is filtered off and the solution is evaporated in vacuo to an oil which is dissolved in EtOAc and washed with 2% NaHCO₃, 2% KHSO₄ and H₂ O. After drying with Na₂ SO₄ the EtOAc phase is evaporated in vacuo and the substance is precipitated with diethylether as an oil which solidifies in vacuo.

Yield: 73%

TLC: Rf=0.55 (P₁).

2b) α-Boc-O-Bzl-Ser-Gly-ε-Z-Lys,pNA Molecular weight=734.2

10 ml TFA is added to 5 mmol α-Boc-Gly-ε-Z-Lys-pNA (2a) dissolved in 20 ml methylenchloride. The mixture is stirred for 30 minutes at room temperature and the substance is precipitated with diethyleter. The dry substance is dissolved in 25 ml DMF and neutralized in cold (-10° C.) with Et₃ N. To the solution 5 mmol α-Boc-O-Bzl-Ser-OH, 5 mmol HOBT and 5.1 mmol DCCl are added. The mixture is stirred for 1 hour in cold and overnight at room temperature. The formed DCU is filtered off and the solution is evaporated in vacuo to an oil which is dissolved in EtOAc and washed with 2% NaHCO₃, 2% KHSO₄ and H₂ O. After drying with Na₂ SO₄ the EtOAc phase is evaporated and the substance is precipitated with diethylether as an oil which solidifies in vacuo.

Yield: 86%

TLC: Rf=0.62 (P₁).

2c) α-Boc-Ile-O-Bzl-Ser-Gly-ε-Z-Lys-pNA Molecular weight:=848.0

10 ml TFA is added to 2 mmol α-Boc-O-Bzl-Ser-Gly-ε-Z-Lys-pNA (2b) dissolved in 20 ml methylenchloride. The mixture is stirred for 30 minutes at room temperature and the substance is precipitated with diethylether. The dry substance is dissolved in 25 ml DMF and neutralized in cold (-10° C.) with Et₃ N. To the solution 2 mmol α-Boc-Ile-ONp is added. The mixture is stirred for 1 hour in cold and 48 hours at room temperature. The solution is evaporated in vacuo to an oil which is dissolved in EtOAc and washed with 2% NaHCO₃, 2% KHSO₄ and H₂ O. After drying with Na₂ SO₄ the EtOAc phase is evaporated and the substance is precipitated with diethylether.

Yield: 72%

TLC: Rf=0.62 (P₁).

2d) α-Ac-Ile-O-Bzl-Ser-Gly-ε-Z-Lys-pNA Molecular weight=789.9

5 ml TFA is added to 1 mmol α-Boc-Ile-O-Bzl-Ser-Gly-ε-Z-Lys-pNA (2c) dissolved in 10 ml methylenchloride. The mixture is stirred for 30 minutes at room temperature and the substance is precipitated with diethylether. The dry substance is dissolved in 10 ml DMF and neutralized in cold (-10° C.) with 80 μl Et₃ N. 70 μl acetic anhydride is added ad the mixture is stirred for 1 hour in cold and 2 hours at room temperature. The solution is evaporated in vacuo to an oil and the substance is precipitated with water.

Yield: 85%

TLC: Rf=0.55 (P₁).

2) α-Ac-Ile-Ser-Gly-Lys-pNA.HCl Molecular weight=602.11

10 ml triflic acid is added to a cold (-10° C.) suspension of 1 mmol α-Ac-Ile-O-Bzl-Ser-Gly-ε-Z-Lys-pNA (2d) in 10 ml methylenchloride. The mixture is stirred for 50 minutes at room temperature and precipitated with diethylether. The product is ion exchanged and purified in the same way as in example 1.

Yield: 35%

TLC: Rf=0.2 (Pa₆)

HPLC: 97% purity.

[α]_(D) ²⁵ =-50.1° (c=0.5%)

EXAMPLE 3

α-Ac-Ile-Thr-Gly-Lys-pNA.HCl Molecular weight=616.13

3a) α-Boc-O-Bzl-Thr-Gly-ε-Lys-pNA Molecular weight=748.2

10 ml TFA is added to 5 mmol α-Boc-Gly-ε-Z-Lys-pNA (2a) dissolved in 20 ml methylenchloride. The mixture is stirred for 30 minutes at room temperature and the substance is precipitated with diethylether. The dry substance is dissolved in 25 ml DMF and neutralized in cold (-10° C.) with Et₃ N. To the solution 5 mmol α-Boc-O-Bzl-Thr-OH, 5 mmol HOBT and 5.1 mmol DCCl are added. The mixture is stirred for 1 hour in cold and overnight at room temperature. The formed DCU is filtered off and the solution is evaporated in vacuo to an oil, which is dissolved in EtOAc and washed with 2% NaHCO₃, 2% KHSO₄ and H₂ O. After drying with Na₂ SO₄ the EtOAc phase is evaporated and the substance is precipitated with diethylether as an oil which solidifies in vacuo.

Yield: 51%

TLC: Rf=0.65 (P₁).

3b) α-Boc-Ile-O-Bzl-Thr-Gly-ε-Z-Lys-pNA Molecular weight=862.0

10 ml TFA is added to 2 mmol α-Boc-O-Bzl-Thr-Gly-ε-Z-Lys-pNA (3a) dissolved in 20 ml methylenchloride. The mixture is stirred for 30 minutes at room temperature and the substance is precipitated with diethylether. The dry substance is dissolved in 25 ml DMF and neutralized in cold (-10° C.) with Et₃ N. To the solution 2 mmol α-Boc-Ile-ONp is added. The mixture is stirred for 1 hour in cold and 48 hours at room temperature. The solution is evaporated in vacuo to an oil which is dissolved in EtOAc and washed with 2% NaHCO₃, 2% KHSO₄ and H₂ O. After drying with Na₂ SO₄ the EtOAc phase is evaporated and the substance is precipitated with diethylether.

Yield: 50%

TLC: Rf=0.7 (P₁)

3c) α-Ac-Ile-O-Bzl-Thr-Gly-ε-Z-Lys-pNA Molecular weight=803.9

5 ml TFA is added to 1 mmol α-Boc-Ile-O-Bzl-Thr-Gly-ε-Z-Lys-pNA (3b) dissolved in 10 ml methylenchloride. The mixture is stirred for 30 minutes at room temperature and the substance is precipitated with diethylether. The dry substance is dissolved in 10 ml DMF and neutralized in cold (-10° C.) with 80 μl Et₃ N. 70 μl acetic anhydride is added and the mixture is stirred for 1 hour in cold (-10° C.) and 2 hours at room temperature. The solution is evaporated in vacuo to an oil and the substance is precipitated in water.

Yield: 86%

TLC: Rf=0.55 (P₁)

3) Ac-Ile-Thr-Gly-Lys-pNA.HCl Molecular weight=616.13

10 ml triflic acid is added to a cold (-10° C.) suspension of 1 mmol α-Ac-Ile-O-Bzl-Thr-Gly-ε-Z-Lys-pNA (3c) in 10 ml methylenchloride. The mixture is stirred for 50 minutes at room temperature and precipitated with diethylether. The product is ion exchanged and purified in the same way as in example 1.

Yield: 36%

TLC: Rf=0.21 (Pa₆)

HPLC: 97% purity

[α]_(D) ²⁵ =-53.2° (c=0.3%)

EXAMPLE 4

α-Ac-Ile-Glu-Gly-Arg-pNA.HCl Molecular weight=673.18

4a) Glyc-Arg-pNA.HCl Molecular weight:=388.8

5 mmol Arg-pNA.2 HBr dissolved in 30 ml DMF is neutralized in cold (-10° C.) with Et₃ N. To the solution 5 mmol glycolic acid, 5 mmol HOBT and 5.1 mmol DCCl are added. The mixture is stirred for 1 hour in cold and 72 hours at room temperature. The formed DCU is filtered off and the solution is evaporated in vacuo to an oil, which is purified on a Sephadex® QAE-25 column in chloride form with 90% ETOH as eluent.

Yield: 74%

TLC: Rf=0.35 (A).

4b) α-Boc-γ-O-Bzl-Glu-Glyc-Arg-pNA.HCl Molecular weight=708.2

To 5 mmol Glyc-Arg-pNA.HCl dissolved in 30 ml DMF and cooled to -10° C., 5 mmol α-Boc-γ-O-Bzl-Glu-OH, 5 mmol HOBT, 0.5 mmol DMAP and 5.1 mmol DCCl are added. The mixture is stirred for 1 hour in cold and 48 hours at room temperature. The formed DCU is filtered off and the solution is evaporated in vacuo to an oil which is dissolved in EtOAc and washed with 2% NaHCO₃, 2% KHSO₄ and H₂ O. After drying with Na₂ SO₄, the EtOAc is evaporated and the substance is precipitated with diethylether.

Yield: 66%

TLC: Rf=0.35 (Pa₆).

4c) α-Boc-Ile-γ-O-Bzl-Glu-Glyc-Arg-pNA.HCl Molecular weight=821.3

10 ml TFA is added to 2 mmol α-Boc-γ-O-Bzl-Glu-Glyc-Arg-pNA.HCl (4b) dissolved in 20 ml methylenchloride. The mixture is stirred for 30 minutes at room temperature and the substance is precipitated with diethyleter. The dry substance is dissolved in 25 ml DMF and neutralized in cold (-10° C.) with Et₃ N. To the solution 2 mmol α-Boc-Ile-ONp is added. The mixture is stirred for 1 hour in cold and 48 hours at room temperature. The solution is evaporated in vacuo to an oil which is dissolved in EtOAc and washed with 2% NaHCO₃, 2% KHSO₄ and H₂ O. After drying with Na₂ SO₄ the EtOAc phase is evaporated and the substance is precipitated with diethylether.

Yield: 69%

TLC: Rf=0.43 (Pa₆)

4d) α-Ac-Ile-γ-O-Bzl-Glu-Glyc-Arg-pNA.HCl Molecular weight=763.3

5 ml TFA is added to 1 mmol α-Boc-Ile-γ-O-Bzl-Glu-Glyc-Arg-pNA.HCl (4c) dissolved in 10 ml methylenchloride. The mixture is stirred for 30 minutes at room temperature and the substance is precipitated with diethylether. The dried substance is dissolved in 10 ml DMF and neutralized in cold (-10° C.) with 130 μl Et₃ N. 120 μl acetic anhydride is added and the mixture is stirred for 1 hour in cold and 2 hours at room temperature. The solution is evaporated in vacuo to an oil and the substance is precipitated with H₂ O.

Yield: 94%

TLC: Rf=0.34 (Pa₆).

4α-Ac-Ile-Glu-Glyc-Arg-pNA.HCl Molecular weight=673.18

10 ml triflic acid is added to a cold (-10° C.) suspension of 1 mmol α-Ac-Ile-γ-O-Bzl-Glu-Glyc-Arg-pNA.HCl (4d) in 10 ml methylenchloride. The mixture is stirred for 50 minutes at room temperature and precipitated with diethylether. The substance is ion exchanged and purified in the same way as in example 1.

Yield: 31%

TLC: Rf=0.45 (A)

HPLC: 98% purity

[α]_(D) ²⁵ =-60.6° (c=0.4%)

Comparison of the Chromogenic Substrates Using a Single-stage Chromogenic LAL-test.*

A series of endotoxin containing solutions, making up a standard curve in the range 0.1-1.2 EU/ml, were assayed as described below using the different chromogenic substrates. The slopes of the resulting standard curves were compared and the slope for the substrate S-2423 was considered to be 100%.

A reagent (100 μl) consisting of a mixture of LAL (50% of the clotting concentration) and the chromogenic substrate (4.4 mM) in a Tris buffer of pH 7.9 is added to an equal volume of the sample. The resulting reaction (activation of the LAL and the subsequent hydrolyses of the chromogenic substrate)is carried out at 37° C. After 15 min. the reaction is terminated by the addition of 400 μl of acetic acid and the absorbance is read at 405 nm.

Table I shows the results for the activated LAL with substrates as measured by the concentration of hydrolized pNA. Substrate S-2423 (Ac-Ile-Glu-Gly-Arg-pNA.HCl) is taken as standard.

                  TABLE I                                                          ______________________________________                                         Screening of substrate for single stage method - endotoxin with                substrate concentration 2.2 mM (in the reaction solution).                     So = 2.2 mM                                                                                                    Relative                                       Substrate                                                                              Peptide sequence        activity                                       ______________________________________                                         S-2423  Ac--Ile--Glu--Gly--Arg--pNA.HCl                                                                        100                                            S-2834  Ac--Ile--Glu--Gly--Lys--pNA.HCl                                                                        126                                            S-2854  Ac--Ile--Ser--Gly--Lys--pNA.HCl                                                                        131                                            S-2860  Ac--Ile--Glu-Glycolyl-Arg--pNA.HCl                                                                     142                                            ______________________________________                                    

This table clearly shows that when using -Gly-Lys or Glyc-Arg as carboxy terminal sequence in the substrate, the activity is surprisingly considerably higher than when the known substrate S-2423, which has Gly-Arg as carboxyterminal sequence, is used.

    __________________________________________________________________________     SEQUENCE LISTING                                                               (1) GENERAL INFORMATION:                                                       (iii) NUMBER OF SEQUENCES: 34                                                  (2) INFORMATION FOR SEQ ID NO:1:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 6 amino acids                                                      (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: peptide                                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Peptide                                                          (B) LOCATION: 1..6                                                             (D) OTHER INFORMATION: /label=Page-2                                           /note="chromogenic synthetic paptide or peptide                                isostere derivative"                                                           (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 1                                                                (D) OTHER INFORMATION: /label=R1                                               /note="H or a protective group"                                                (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B ) LOCATION: 2                                                               (D) OTHER INFORMATION: /label=A1                                               /note="H, Ile, Leu, or Val"                                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 3                                                                (D) OTHER INFORMATION: /label=A2                                               /note="Glu, Asp, Ser, or Thr"                                                  (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 4                                                                (D) OTHER INFORMATION: /label=A3                                               /note="Gly or Glyc(glycolic acid)"                                             (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 5                                                                (D) OTHER INFORMATION: /label=A4                                               /note="Arg or Lys"                                                             (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 6                                                                (D) OTHER INFORMATION: /label=R2                                                /note="4-nitro aniline"                                                       (xi) SEQUENCE DESCRIPTION: SEQ ID NO:1:                                        XaaXaaXaaXaaXaaXaa                                                             15                                                                             (2) INFORMATION FOR SEQ ID NO:2:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 6 amino acids                                                      (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: peptide                                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Peptide                                                           (B) LOCATION: 1..6                                                            (D) OTHER INFORMATION: /label=Page-3                                           /note="chromogenic synthetic peptide or peptide                                isostere derivative"                                                           (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 1                                                                (D) OTHER INFORMATION: /label=R1                                               /note="H or a protective group"                                                (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 2                                                                (D) OTHER INFORMATION: /label=A1                                               /note="H, Ile, Leu, or Val"                                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 3                                                                (D) OTHER INFORMATION: /label=A2                                               /note="Glu, Asp, Ser, or Thr"                                                  (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 4                                                                 (D) OTHER INFORMATION: /label=A3                                              /note="Gly or Glyc(glycolic acid)"                                             (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 5                                                                (D) OTHER INFORMATION: /label=A4                                               /note="Arg or Lys"                                                             (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 6                                                                (D) OTHER INFORMATION: /label=R2                                                /note="an aromatic or hetrerocyclic group which                               gives the compound R2-NH2 by enzymatice                                        hydrolysis"                                                                    (xi) SEQUENCE DESCRIPTION: SEQ ID NO:2:                                        XaaXaaXaaXaaXaaXaa                                                             15                                                                             (2) INFORMATION FOR SEQ ID NO:3:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 7 amino acids                                                      (B) TYPE: amino acid                                                            (D) TOPOLOGY: linear                                                          (ii) MOLECULE TYPE: peptide                                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Peptide                                                          (B) LOCATION: 1..7                                                             (D) OTHER INFORMATION: /label=Example-1                                        (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 1                                                                (D) OTHER INFORMATION: /note="alpha-acetyl"                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                     (B) LOCATION: 6                                                               (D) OTHER INFORMATION: /note="p-nitroaniline"                                  (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 7                                                                (D) OTHER INFORMATION: /note="trifluoroacetic acid"                            (xi) SEQUENCE DESCRIPTION: SEQ ID NO:3:                                        XaaIleGluGlyLysXaaXaa                                                          15                                                                             (2) INFORMATION FOR SEQ ID NO:4:                                               (i) SEQUENCE CHARACTERISTICS:                                                   (A) LENGTH: 4 amino acids                                                     (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: peptide                                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Peptide                                                          (B) LOCATION: 1..4                                                             (D) OTHER INFORMATION: /label=Example-1a                                       (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 1                                                                (D) OTHER INFORMATION: /note="epsilon-benzyloxycarbonyl"                        (ix) FEATURE:                                                                 (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 3                                                                (D) OTHER INFORMATION: /note="p-nitroaniline"                                  (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 4                                                                (D) OTHER INFORMATION: /note="trifluoroacetic acid"                            (xi) SEQUENCE DESCRIPTION: SEQ ID NO:4:                                        XaaLysXaaXaa                                                                   (2) INFORMATION FOR SEQ ID NO:5:                                               (i ) SEQUENCE CHARACTERISTICS:                                                 (A) LENGTH: 6 amino acids                                                      (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: peptide                                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Peptide                                                          (B) LOCATION: 1..5                                                             (D) OTHER INFORMATION: /label=Example-1b                                       (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 1                                                                (D) OTHER INFORMATION: /note= "alpha-t-butyloxycarbonyl"                       (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 2                                                                (D) OTHER INFORMATION: /note="gamma-Oxygen"                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 3                                                                (D) OTHER INFORMATION: /note="benzyl"                                          (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 6                                                                (D) OTHER INFORMATION: /note="-OH group"                                       (xi) SEQUENCE DESCRIPTION: SEQ ID NO:5:                                        XaaXaaXaaGluGlyXaa                                                             15                                                                             (2) INFORMATION FOR SEQ ID NO:6:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 7 amino acids                                                      (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: peptide                                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Peptide                                                           (B) LOCATION: 1..7                                                            (D) OTHER INFORMATION: /label=Example-1c                                       (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 1                                                                (D) OTHER INFORMATION: /note="Hydrogen"                                        (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 2                                                                (D) OTHER INFORMATION: /note="gamma-Oxygen"                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                     (B) LOCATION: 3                                                               (D) OTHER INFORMATION: /note="benzyl"                                          (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 6                                                                (D) OTHER INFORMATION: /note="-OH group"                                       (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 7                                                                (D) OTHER INFORMATION: /note="trifluoroacetic acid"                            (xi) SEQUENCE DESCRIPTION: SEQ ID NO:6:                                        XaaX aaXaaGluGlyXaaXaa                                                         15                                                                             (2) INFORMATION FOR SEQ ID NO:7:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 7 amino acids                                                      (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: peptide                                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Peptide                                                          (B) LOCATION: 1..7                                                             (D) OTHER INFORMATION: /label=Example-1d                                       (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 1                                                                (D) OTHER INFORMATION: /note="alpha-t-butyloxycarbonyl"                        (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 3                                                                (D) OTHER INFORMATION: /note="gamma-Oxygen"                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 4                                                                (D) OTHER INFORMATION: /note="benxyl"                                          (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 7                                                                (D) OTHER INFORMATION: /note="-OH group"                                       (xi) SEQUENCE DESCRIPTION: SEQ ID NO:7:                                        XaaIleXaaXaaGluGlyXaa                                                          15                                                                             (2) INFORMATION FOR SEQ ID NO:8:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 9 amino acids                                                       (B) TYPE: amino acid                                                          (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: peptide                                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Peptide                                                          (B) LOCATION: 1..9                                                             (D) OTHER INFORMATION: /label=Example-1e                                       (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 1                                                                (D) OTHER INFORMATION: /note="alpha-t-butyloxycarbonyl"                        (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 3                                                                (D) OTHER INFORMATION: /note="gamma-Oxygen"                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 4                                                                (D) OTHER INFORMATION: /note="benzyl"                                          (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 7                                                                (D) OTHER INFORMATION: /note="epsilon-benzyloxycarbonyl"                       (ix) FEATURE:                                                                  (A ) NAME/KEY: Modified-site                                                   (B) LOCATION: 9                                                                (D) OTHER INFORMATION: /note="p-nitroaniline"                                  (xi) SEQUENCE DESCRIPTION: SEQ ID NO:8:                                        XaaIleXaaXaaGluGlyXaaLysXaa                                                    15                                                                             (2) INFORMATION FOR SEQ ID NO:9:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 9 amino acids                                                      (B) TYPE: amino acid                                                           (D ) TOPOLOGY: linear                                                          (ii) MOLECULE TYPE: peptide                                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Peptide                                                          (B) LOCATION: 1..9                                                             (D) OTHER INFORMATION: /label=Example-1f                                       (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 1                                                                (D) OTHER INFORMATION: /note="alpha-acetyl"                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 3                                                                 (D) OTHER INFORMATION: /note="gamma-Oxygen"                                   (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 4                                                                (D) OTHER INFORMATION: /note="benzyl"                                          (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 7                                                                (D) OTHER INFORMATION: /note="epsilon-benzyloxycarbonyl"                       (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 9                                                                 (D) OTHER INFORMATION: /note="p-nitroaniline"                                 (xi) SEQUENCE DESCRIPTION: SEQ ID NO:9:                                        XaaIleXaaXaaGluGlyXaaLysXaa                                                    15                                                                             (2) INFORMATION FOR SEQ ID NO:10:                                              (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 7 amino acids                                                      (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: peptide                                                     (ix) FEATURE:                                                                 (A) NAME/KEY: Peptide                                                          (B) LOCATION: 1..7                                                             (D) OTHER INFORMATION: /label=Example-2                                        (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 1                                                                (D) OTHER INFORMATION: /note="alpha-acetyl"                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 6                                                                (D) OTHER INFORMATION: /note="p-nitroaniline"                                  ( ix) FEATURE:                                                                 (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 7                                                                (D) OTHER INFORMATION: /note="HCl"                                             (xi) SEQUENCE DESCRIPTION: SEQ ID NO:10:                                       XaaIleSerGlyLysXaaXaa                                                          15                                                                             (2) INFORMATION FOR SEQ ID NO:11:                                              (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 5 amino acids                                                      (B) TYPE: amino acid                                                            (D) TOPOLOGY: linear                                                          (ii) MOLECULE TYPE: peptide                                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Peptide                                                          (B) LOCATION: 1..5                                                             (D) OTHER INFORMATION: /label=Example-2a                                       (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 1                                                                (D) OTHER INFORMATION: /note="alpha-t-butyloxycarbonyl"                        (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                     (B) LOCATION: 3                                                               (D) OTHER INFORMATION: /note="epsilon-benzyloxycarbonyl"                       (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 5                                                                (D) OTHER INFORMATION: /note="p-nitroaniline"                                  (xi) SEQUENCE DESCRIPTION: SEQ ID NO:11:                                       XaaGlyXaaLysXaa                                                                15                                                                             (2) INFORMATION FOR SEQ ID NO:12:                                              (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 8 amino acids                                                      (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: peptide                                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Peptide                                                          (B) LOCATION: 1..8                                                             (D) OTHER INFORMATION: /label=Example-2b                                       (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 1                                                                (D) OTHER INFORMATION: /note="alpha-t-butyloxycarbonyl"                        (ix) FEATURE:                                                                   (A) NAME/KEY: Modified-site                                                   (B) LOCATION: 2                                                                (D) OTHER INFORMATION: /note="Oxygen"                                          (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 3                                                                (D) OTHER INFORMATION: /note="benzyl"                                          (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 6                                                                (D) OTHER INFORMATION: /note="epsilon-benzyloxycarbonyl"                       (ix) FEATURE:                                                                   (A) NAME/KEY: Modified-site                                                   (B) LOCATION: 8                                                                (D) OTHER INFORMATION: /note="p-nitroaniline"                                  (xi) SEQUENCE DESCRIPTION: SEQ ID NO:12:                                       XaaXaaXaaSerGlyXaaLysXaa                                                       15                                                                             (2) INFORMATION FOR SEQ ID NO:13:                                              (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 9 amino acids                                                      (B) TYPE: amino acid                                                            (D) TOPOLOGY: linear                                                          (ii) MOLECULE TYPE: peptide                                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Peptide                                                          (B) LOCATION: 1..9                                                             (D) OTHER INFORMATION: /label=Example-2c                                       (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 1                                                                (D) OTHER INFORMATION: /note="alpha-t-butyloxycarbonyl"                        (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B ) LOCATION: 3                                                               (D) OTHER INFORMATION: /note="Oxygen"                                          (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 4                                                                (D) OTHER INFORMATION: /note="benzyl"                                          (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 7                                                                (D) OTHER INFORMATION: /note="epsilon-benzyloxycarbonyl"                       (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    ( B) LOCATION: 9                                                               (D) OTHER INFORMATION: /note="p-nitroaniline"                                  (xi) SEQUENCE DESCRIPTION: SEQ ID NO:13:                                       XaaIleXaaXaaSerGlyXaaLysXaa                                                    15                                                                             (2) INFORMATION FOR SEQ ID NO:14:                                              (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 9 amino acids                                                      (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: peptide                                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Peptide                                                          (B) LOCATION: 1..9                                                             (D) OTHER INFORMATION: /label=Example-2d                                       (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 1                                                                (D) OTHER INFORMATION: /note="alpha-acetyl"                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 3                                                                (D) OTHER INFORMATION: /note="Oxygen"                                          (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 4                                                                (D) OTHER INFORMATION: /note="benzyl"                                          (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 7                                                                (D) OTHER INFORMATION: /note="epsilon-benzyloxycarbonyl"                       (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 9                                                                (D) OTHER INFORMATION: /note="p-nitroaniline"                                  (xi) SEQUENCE DESCRIPTION: SEQ ID NO:14:                                       XaaIleXaaXaaSerGlyXaaLysXaa                                                    15                                                                             (2) INFORMATION FOR SEQ ID NO:15:                                              (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 7 amino acids                                                      (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: peptide                                                    (ix) FEATURE:                                                                   (A) NAME/KEY: Peptide                                                         (B) LOCATION: 1..7                                                             (D) OTHER INFORMATION: /label=Example-3                                        (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 1                                                                (D) OTHER INFORMATION: /note="alpha-acetyl"                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 6                                                                (D) OTHER INFORMATION: /note="p-nitroaniline"                                  (ix) FEATURE:                                                                   (A) NAME/KEY: Modified-site                                                   (B) LOCATION: 7                                                                (D) OTHER INFORMATION: /note="HCl"                                             (xi) SEQUENCE DESCRIPTION: SEQ ID NO:15:                                       XaaIleThrGlyLysXaaXaa                                                          15                                                                             (2) INFORMATION FOR SEQ ID NO:16:                                              (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 7 amino acids                                                      (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                            (ii) MOLECULE TYPE: peptide                                                   (ix) FEATURE:                                                                  (A) NAME/KEY: Peptide                                                          (B) LOCATION: 1..7                                                             (D) OTHER INFORMATION: /label=Example-3a                                       (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 1                                                                (D) OTHER INFORMATION: /note="alpha-t-butyloxycarbonyl"                        (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 2                                                                 (D) OTHER INFORMATION: /note="Oxygen"                                         (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 3                                                                (D) OTHER INFORMATION: /note="benzyl"                                          (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 7                                                                (D) OTHER INFORMATION: /note="p-nitroaniline"                                  (xi) SEQUENCE DESCRIPTION: SEQ ID NO:16:                                       XaaXaaXaaThrGlyLysXaa                                                           15                                                                            (2) INFORMATION FOR SEQ ID NO:17:                                              (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 9 amino acids                                                      (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: peptide                                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Peptide                                                          (B) LOCATION: 1..9                                                             (D) OTHER INFORMATION: /label=Example-3b                                       (ix) FEATURE:                                                                   (A) NAME/KEY: Modified-site                                                   (B) LOCATION: 1                                                                (D) OTHER INFORMATION: /note="alpha-t-butyloxycarbonyl"                        (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 3                                                                (D) OTHER INFORMATION: /note="Oxygen"                                          (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 4                                                                (D) OTHER INFORMATION: /note="benzyl"                                          (ix) FEATURE:                                                                   (A) NAME/KEY: Modified-site                                                   (B) LOCATION: 7                                                                (D) OTHER INFORMATION: /note="epsilon-benzyloxycarbonyl"                       (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 9                                                                (D) OTHER INFORMATION: /note="p-nitroaniline"                                  (xi) SEQUENCE DESCRIPTION: SEQ ID NO:17:                                       XaaIleXaaXaaThrGlyXaaLysXaa                                                    15                                                                             ( 2) INFORMATION FOR SEQ ID NO:18:                                             (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 9 amino acids                                                      (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: peptide                                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Peptide                                                          (B) LOCATION: 1..9                                                             (D) OTHER INFORMATION: /label=Example-3c                                       (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 1                                                                 (D) OTHER INFORMATION: /note="alpha-acetyl"                                   (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 3                                                                (D) OTHER INFORMATION: /note="Oxygen"                                          (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 4                                                                (D) OTHER INFORMATION: /note="benzyl"                                          (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 7                                                                (D) OTHER INFORMATION: /note="epsilon-benzyloxycarbonyl"                       (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 9                                                                (D) OTHER INFORMATION: /note="p-nitoraniline"                                  (xi) SEQUENCE DESCRIPTION: SEQ ID NO:18:                                       XaaIleXaaXaaThrGlyXaaLysXaa                                                    15                                                                             (2) INFORMATION FOR SEQ ID NO:19:                                              (i) SEQUENCE CHARACTERISTICS:                                                   (A) LENGTH: 7 amino acids                                                     (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: peptide                                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Peptide                                                          (B) LOCATION: 1..7                                                             (D) OTHER INFORMATION: /label=Example-4                                        (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 1                                                                (D) OTHER INFORMATION: /note="alpha-acetyl"                                     (ix) FEATURE:                                                                 (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 4                                                                (D) OTHER INFORMATION: /note="glycolic acid"                                   (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 6                                                                (D) OTHER INFORMATION: /note="p-nitroaniline"                                  (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 7                                                                (D) OTHER INFORMATION: /note="HCl"                                              (xi) SEQUENCE DESCRIPTION: SEQ ID NO:19:                                      XaaIleGluXaaArgXaaXaa                                                          15                                                                             (2) INFORMATION FOR SEQ ID NO:20:                                              (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 4 amino acids                                                      (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: peptide                                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Peptide                                                           (B) LOCATION: 1..4                                                            (D) OTHER INFORMATION: /label=Example-4a                                       (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 1                                                                (D) OTHER INFORMATION: /note="glycolic acid"                                   (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 3                                                                (D) OTHER INFORMATION: /note="p-nitroaniline"                                  (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                     (B) LOCATION: 4                                                               (D) OTHER INFORMATION: /note="HCl"                                             (xi) SEQUENCE DESCRIPTION: SEQ ID NO:20:                                       XaaArgXaaXaa                                                                   1                                                                              (2) INFORMATION FOR SEQ ID NO:21:                                              (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 8 amino acids                                                      (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: peptide                                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Peptide                                                          (B) LOCATION: 1..8                                                             (D) OTHER INFORMATION: /label=Example-4b                                       (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 1                                                                (D) OTHER INFORMATION: /note="alpha-t-butyloxycarbonyl"                        (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 2                                                                (D) OTHER INFORMATION: /note="gamma-O"                                         (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 3                                                                (D) OTHER INFORMATION: /note="benzyl"                                          (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 5                                                                (D) OTHER INFORMATION: /note="glycolic acid"                                   (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 7                                                                (D) OTHER INFORMATION: /note="p-nitroaniline"                                  (ix) FEATURE:                                                                  (A ) NAME/KEY: Modified-site                                                   (B) LOCATION: 8                                                                (D) OTHER INFORMATION: /note="HCl"                                             (xi) SEQUENCE DESCRIPTION: SEQ ID NO:21:                                       XaaXaaXaaGluXaaArgXaaXaa                                                       15                                                                             (2) INFORMATION FOR SEQ ID NO:22:                                              (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 9 amino acids                                                      (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: peptide                                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Peptide                                                          (B) LOCATION: 1..9                                                             (D) OTHER INFORMATION: /label=Example-4c                                       (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 1                                                                (D) OTHER INFORMATION: /note="alpha-t-butyloxycarbonyl"                        (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 3                                                                 (D) OTHER INFORMATION: /note="gamma-O"                                        (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 4                                                                (D) OTHER INFORMATION: /note="benzyl"                                          (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 6                                                                (D) OTHER INFORMATION: /note="glycolic acid"                                   (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 8                                                                 (D) OTHER INFORMATION: /note="p-nitroaniline"                                 (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 9                                                                (D) OTHER INFORMATION: /note="HCl"                                             (xi) SEQUENCE DESCRIPTION: SEQ ID NO:22:                                       XaaIleXaaXaaGluXaaArgXaaXaa                                                    15                                                                             (2) INFORMATION FOR SEQ ID NO:23:                                              (i) SEQUENCE CHARACTERISTICS:                                                   (A) LENGTH: 9 amino acids                                                     (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: peptide                                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Peptide                                                          (B) LOCATION: 1..9                                                             (D) OTHER INFORMATION: /label=Example-4d                                       (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 1                                                                (D) OTHER INFORMATION: /note="alpha-acetyl"                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 3                                                                (D) OTHER INFORMATION: /note="gamma-O"                                         (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 4                                                                (D) OTHER INFORMATION: /note="benzyl"                                          (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 6                                                                (D) OTHER INFORMATION: /note="glycolic acid"                                   (i x) FEATURE:                                                                 (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 8                                                                (D) OTHER INFORMATION: /note="p-nitroaniline"                                  (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 9                                                                (D) OTHER INFORMATION: /note="HCl"                                             (xi) SEQUENCE DESCRIPTION: SEQ ID NO:23:                                       XaaIleXaaXaaGluXaaArgXaaXaa                                                    1 5                                                                            (2) INFORMATION FOR SEQ ID NO:24:                                              (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 7 amino acids                                                      (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: peptide                                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Peptide                                                          (B) LOCATION: 1..7                                                             (D) OTHER INFORMATION: /label=Table-1-seq-1                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                     (B) LOCATION: 1                                                               (D) OTHER INFORMATION: /note="acetyl"                                          (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 6                                                                (D) OTHER INFORMATION: /note="p-nitroaniline"                                  (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 7                                                                (D) OTHER INFORMATION: /note="HCl"                                             (xi) SEQUENCE DESCRIPTION: SEQ ID NO:24:                                       XaaIleG luGlyArgXaaXaa                                                         15                                                                             (2) INFORMATION FOR SEQ ID NO:25:                                              (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 6 amino acids                                                      (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: peptide                                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Peptide                                                          (B) LOCATION: 1..6                                                             (D) OTHER INFORMATION: /label=Claim-16                                         (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 1                                                                (D) OTHER INFORMATION: /label=R1                                               /note="H or a protective group"                                                (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 2                                                                (D) OTHER INFORMATION: /label=A1                                               /note="H, Ile, Leu, or Val"                                                    (ix) FEATURE:                                                                   (A) NAME/KEY: Modified-site                                                   (B) LOCATION: 3                                                                (D) OTHER INFORMATION: /label=A2                                               /note="Glu, Asp, Ser, or Thr"                                                  (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 4                                                                (D) OTHER INFORMATION: /label=A3                                               /note="Gly or Glyc(glycolic acid)"                                             (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                     (B) LOCATION: 5                                                               (D) OTHER INFORMATION: /label=A4                                               /note="Arg or Lys"                                                             (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 6                                                                (D) OTHER INFORMATION: /label=R2                                               /note="a group that upon enzymatic hydrolysis                                  gives para- nitroaniline(pNA), with the proviso                                that A3 is Gly when A4 is Lys and Glyc when A4 is                               (xi) SEQUENCE DESCRIPTION: SEQ ID NO:25:                                      XaaXaaXaaXaaXaaXaa                                                             15                                                                             (2) INFORMATION FOR SEQ ID NO:26:                                              (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 6 amino acids                                                      (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: peptide                                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Peptide                                                           (B) LOCATION: 1..6                                                            (D) OTHER INFORMATION: /label=Claim-18a                                        (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 1                                                                (D) OTHER INFORMATION: /note="alpha-acetal"                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 6                                                                (D) OTHER INFORMATION: /note="p-nitroaniline"                                  (xi) SEQUENCE DESCRIPTION: SEQ ID NO:26:                                       XaaIleGluGlyL ysXaa                                                            15                                                                             (2) INFORMATION FOR SEQ ID NO:27:                                              (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 6 amino acids                                                      (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: peptide                                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Peptide                                                          (B) LOCATION: 1..6                                                             (D) OTHER INFORMATION: /label=Claim-18b                                        (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 1                                                                (D) OTHER INFORMATION: /note="alpha-acetyl"                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 6                                                                (D) OTHER INFORMATION: /note="p-nitroaniline"                                  (xi) SEQUENCE DESCRIPTION: SEQ ID NO:27:                                       XaaIleSerGlyLysXaa                                                             15                                                                             (2) INFORMATION FOR SEQ ID NO:28:                                              (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 6 amino acids                                                      (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: peptide                                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Peptide                                                          (B) LOCATION: 1..6                                                             (D) OTHER INFORMATION: /label=Claim-18c                                        (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 1                                                                 (D) OTHER INFORMATION: /note="alpha-acetyl"                                   (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 6                                                                (D) OTHER INFORMATION: /note="p-nitroaniline"                                  (xi) SEQUENCE DESCRIPTION: SEQ ID NO:28:                                       XaaIleThrGlyLysXaa                                                             15                                                                             (2) INFORMATION FOR SEQ ID NO:29:                                              (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 6 amino acids                                                      (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: peptide                                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Peptide                                                          (B) LOCATION: 1..6                                                             (D) OTHER INFORMATION: /label=Claim-18d                                        (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 1                                                                (D) OTHER INFORMATION: /note="alpha-acetyl"                                    (ix) FEATURE:                                                                   (A) NAME/KEY: Modified-site                                                   (B) LOCATION: 4                                                                (D) OTHER INFORMATION: /note="Glyc(glycolic acid)"                             (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 6                                                                (D) OTHER INFORMATION: /note="p-nitroaniline"                                  (xi) SEQUENCE DESCRIPTION: SEQ ID NO:29:                                       XaaIleGluXaaArgXaa                                                             15                                                                             (2) INFORMATION FOR SEQ ID NO:30:                                              (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 6 amino acids                                                      (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: peptide                                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Peptide                                                          (B) LOCATION: 1..6                                                             (D) OTHER INFORMATION: /label=Claim-19                                         (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 1                                                                 (D) OTHER INFORMATION: /label=R1                                              /note="H or a protective group"                                                (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 2                                                                (D) OTHER INFORMATION: /label=A1                                               /note="H, Ile, Leu, or Val"                                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 3                                                                (D) OTHER INFORMATION: /label=A2                                                /note="Glu, Asp, Ser, or Thr"                                                 (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 4                                                                (D) OTHER INFORMATION: /label=A3                                               /note="Gly or Glyc(glycolic acid)"                                             (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 5                                                                (D) OTHER INFORMATION: /label=A4                                               /note="Arg or Lys"                                                              (ix) FEATURE:                                                                 (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 6                                                                (D) OTHER INFORMATION: /label=R2                                               /note="An aromatic or heterocyclic group which by                              enzymatic hydrolysis gives a compound R2-NH2 which                             can be determined quantitatively with the proviso                              (xi) SEQUENCE DESCRIPTION: SEQ ID NO:30:                                       XaaXaaXaaXaaXaaXaa                                                             1 5                                                                            (2) INFORMATION FOR SEQ ID NO:31:                                              (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 6 amino acids                                                      (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: peptide                                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Peptide                                                          (B) LOCATION: 1..6                                                             (D) OTHER INFORMATION: /label=Claim-21a                                        (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                     (B) LOCATION: 1                                                               (D) OTHER INFORMATION: /note="alpha-acetyl"                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 6                                                                (D) OTHER INFORMATION: /note="p-nitroaniline"                                  (xi) SEQUENCE DESCRIPTION: SEQ ID NO:31:                                       XaaIleGluGlyLysXaa                                                             15                                                                             (2) INFORMATION FOR SEQ ID NO:32:                                              (i) SEQUENCE CHARACTERISTICS:                                                   (A) LENGTH: 6 amino acids                                                     (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: peptide                                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Peptide                                                          (B) LOCATION: 1..6                                                             (D) OTHER INFORMATION: /label=Claim-21b                                        (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 1                                                                (D) OTHER INFORMATION: /note="alpha-acetyl"                                     (ix) FEATURE:                                                                 (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 6                                                                (D) OTHER INFORMATION: /note="p-nitroaniline"                                  (xi) SEQUENCE DESCRIPTION: SEQ ID NO:32:                                       XaaIleSerGlyLysXaa                                                             15                                                                             (2) INFORMATION FOR SEQ ID NO:33:                                              (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 6 amino acids                                                      (B) TYPE: amino acid                                                            (D) TOPOLOGY: linear                                                          (ii) MOLECULE TYPE: peptide                                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Peptide                                                          (B) LOCATION: 1..6                                                             (D) OTHER INFORMATION: /label=Claim-21c                                        (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 1                                                                (D) OTHER INFORMATION: /note="aplha-acetyl"                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                     (B) LOCATION: 6                                                               (D) OTHER INFORMATION: /note="p-nitroaniline"                                  (xi) SEQUENCE DESCRIPTION: SEQ ID NO:33:                                       XaaIleThrGlyLysXaa                                                             15                                                                             (2) INFORMATION FOR SEQ ID NO:34:                                              (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 6 amino acids                                                      (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: peptide                                                    (ix ) FEATURE:                                                                 (A) NAME/KEY: Peptide                                                          (B) LOCATION: 1..6                                                             (D) OTHER INFORMATION: /label=Claim-21d                                        (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 1                                                                (D) OTHER INFORMATION: /note="alpha-acetyl"                                    (ix) FEATURE:                                                                  (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 4                                                                (D) OTHER INFORMATION: /note="Glyc(glycolic acid)"                             (ix ) FEATURE:                                                                 (A) NAME/KEY: Modified-site                                                    (B) LOCATION: 6                                                                (D) OTHER INFORMATION: /note="p-nitroaniline"                                  (xi) SEQUENCE DESCRIPTION: SEQ ID NO:34:                                       XaaIleGluXaaArgXaa                                                             15                                                                         

I claim:
 1. Peptide derivatives having the general formula:

    R.sub.1 -A.sub.1 -A.sub.2 -A.sub.3 -A.sub.4 -R.sub.2 or its salt

wherein R₁ =H or a protective group A₁ =H, Ile, Leu or Val A₂ =Glu, Asp, Ser, or Thr A₃ =Gly or Glyc A₄ =Arg or Lys R₂ =is a group that upon enzymatic hydrolysis gives para-nitroaniline (pNA),with the proviso that A₃ is Gly when A₄ is Lys and Glyc when A₄ is Arg.
 2. Peptide derivatives according to claim 1 wherein A₁ =Ile.
 3. Peptide derivatives according to claim 1 having the formula selected from the group consisting of:α-Ac-Ile-Glu-Gly-Lys-pNA, α-Ac-Ile-Ser-Gly-Lys-pNA, α-Ac-Ile-Thr-Gly-Lys-pNA, and α-Ac-Ile-Glu-Glyc-Arg-pNA.
 4. A method for quantitatively determining bacterial endotoxins in a sample which comprises incubating the sample with a Limulus clotting enzyme and a substrate for said enzyme, wherein said substrate is a peptide derivative having the following formula:

    R.sub.1 -A.sub.1 -A.sub.2 -A.sub.3 -A.sub.4 -R.sub.2 or its salt

where R₁ =H or a protective group R₁ =H, Ile, Leu or Val A₂ =Glu, Asp, Ser, or Thr A₃ =Gly or Glyc A₄ =Arg or Lys R₂ =an aromatic or heterocyclic group which by enzymatic hydrolysis gives a compound R₂ -NH₂ which can be determined quantitatively,with the proviso that A₃ is Gly when A₄ is Lys and Glyc when A₄ is Arg.
 5. The method of claim 4 when A is Ile.
 6. The method of claim 5 wherein said peptide derivative is selected from the group consisting of:α-Ac-Ile-Glu-Gly-Lys-pNA, α-Ac-Ile-Ser-Gly-Lys-pNA, α-Ac-Ile-Thr-Gly-Lys-pNA, and α-Ac-Ile-Glu-Glyc-Arg-pNA.
 7. The method of claim 4 wherein said clotting enzyme is Limulus Amebocyte Lysate.
 8. The method of claim 4 wherein R₂ -NH₂ is para-nitroaniline. 